1. Field of the Invention
The present invention relates to an inverter device which converts direct-current power to alternating-current power by respectively turning on and off a plurality of switching elements in order to drive a motor, and particularly to the detection of an initial position of a motor.
2. Description of the Related Art
FIG. 1 shows a conventional inverter device.
An inverter device 80 shown in FIG. 1 converts direct-current power of a direct current source 81 such as a battery into alternating-current power in order to drive a three-phase motor 82.
For example, in the inverter 80, three pairs each including two switching elements connected in series to each other are respectively connected to the direct current source 81 in parallel, and respective midpoints of the three pairs of switching elements are connected to corresponding inputs of a U phase, a V phase and a W phase of the three-phase motor 82. And, switching elements provided for respective phases in the inverter device 80 are sequentially turned on and off so that alternating-current power each having phases different by 120 degrees to each other are respectively supplied to the corresponding phases of the three-phase motor 82. Thereby, the three-phase motor 82 is driven.
Also, the respective switching elements for example are PWM (Pulse Width Modulation) controlled based on currents flowing in the respective phases of the three-phases motor 82, and the currents in the respective phases are obtained by current sensors or the like provided on the respective phases.
It is needed to detect an initial position of a rotor (a rotation angle of a rotor with respect to a given phase as a reference) upon starting the three-phase motor 82 as above in order to effectively produce torque of the three-phase motor 82. And for detecting the initial position of the rotor, methods as below are proposed for example.
(1) A method in which by applying six voltage pulses with different electrical angles by 60 degrees to respective phases of the three-phase motor 82, respective currents in the corresponding phases are obtained, based on parallel components of the obtained currents in the respective phases, a general position of a rotor of the three-phase motor 82 is obtained, further, from perpendicular components of the obtained currents in the respective phases, a correlation value for correlating the general position is obtained, and based on the general position and the correlated value, the initial position of the rotor of the three-phase motor 82 is obtained. (See Patent Document 1 for example)
(2) Some of inductances of coils constituting the three-phase motor 82 are measured in advance corresponding to positions of a rotor of the three-phase motor 82, a reference inductance is determined among the measured inductances, voltage pulses in three patterns are applied to the respective phases of the three-phase motor 82, and the initial position of the rotor of the three-phase motor 82 is obtained based on a result of comparison between the reference inductance and the inductances corresponding to the respective patterns. (See Patent Document 2 for example)
However, in both of the above methods (1) and (2), the initial position of the rotor of the three-phase motor 82 is obtained by a calculation which utilizes detected currents in the respective phases of the three-phase motor 82. When the initial position of the rotor is obtained by a calculation, noise components (for example, a current error upon current detection due to a variation of characteristics in devices constituting the inverter such as current sensors, and current noise such as a ripple current or the like generated depending on a timing of turning on switching elements) included in the detected currents in the respective phases are also included in the calculation so that there is a probability that the noise components become more considerable.
And, there is a problem that the initial position of the rotor of the three-phase motor 82 is not detected accurately when the noise components of the detected currents becomes more considerable due to the calculation.
Also, there is a problem that the above methods (1) or (2) can not be used in the case when a failure of detection of the initial position of the rotor is not tolerated.
Further, in the above methods (1) and (2), there is a problem that when calculation amount increases for obtaining a more accurate initial position, the noise components are further amplified as the calculation amount increases.
Patent Document 1
Japanese Patent Application Publication No. 2002-262600 (page 2 to page 13 and FIGS. 1 to 14)
Patent Document 2
Japanese Patent Application Publication No. 2001-136779 (page 2 to page 6 and FIGS. 1 to 6)